The feature that distinguishes gravure printing from other printing methods is its ability to produce pictures with range of tones by varying the thickness of ink which is achieved by means of varying the depth of ink cells on the printing surface of the plate. Because of this characteristic, the gravure-printed matter exhibits ample gradations of sharp definition and offers wide ranges of colors, contrasts and tones.
In order to make the most of these advantages of gravure printing, the paper substrate to be gravure-printed must allow for high fidelity of dot reproduction. However, in the highlights of gravure print, dots often fail to be formed by ink transfer and the resulting print does not have the intended quality. This problem is particularly serious with coated paper which inherently has excellent printing quality. Improvements of dot reproduction on coated paper for gravure printing are of primary concern to the paper industry because the commercial value of the coated paper is largely dependent on its ability to faithfully reproduce dots.
Styrene-butadiene latices have been extensively used as pigment binders in coated paper for gravure printing, but they are not completely satisfactory for the purpose of faithful dot reproduction and their ability in this respect is further decreased if they are used in combination with water-soluble polymers such as starch commonly used as thickeners or water retention aids. Another serious problem that has been encountered in using styrene-butadiene latices as pigment binders is that even if coated paper as manufactured has a dot reproducing ability that is satisfactory for practical purposes, this ability will be considerably deteriorated with a lapse of time.
Acrylic ester-based emulsions are known as pigment binders that ensure faithful dot reproduction and will not undergo any substantial deterioration with a lapse of time. However, coated paper using such emulsions has a low surface strength (i.e., low dry picking resistance) and is apt to foul the surfaces of supercalendering rolls.
As a result of various studies made in order to eliminate these problems with the conventional pigment binders, the present inventors found that by using a pigment binder made of a copolymer with a specified composition, a coated paper suitable for gravure printing can be provided that ensures faithful dot reproduction without undergoing any time-dependent deterioration, which exhibits a superior dry picking resistance, and which will not foul the surfaces of rolls used in supercalendering. The present invention has been accomplished on the basis of this finding.